Since a solder used for joining electronic components to a printed circuit board generally has a principal ingredient of Sn, Sn and O2 in the air may be reacted to form an oxide film by Sn oxides on a surface of the solder based on any storage conditions before the use thereof. By this tendency, when storing it under a high temperature and high humidity environment, thickness of an oxide film formed on its surface has a tendency of becoming thicker and there may be a case where its surface gloss is lost or a yellow discoloration occurs. It is difficult for flux to remove the oxide film from the solder surface during the soldering and the wettability deteriorates if any oxide film remains on the solder surface, so that fusion properties thereof deteriorate, thereby causing any soldering defect to occur.
On the other hand, in recent years, together with development of compact information devices, downsizing of electronic components to be installed on the information device have also rapidly been progressed. In order to meet the downsizing requirement, a ball grid array (hereinafter, referred to as “BGA”), in which electrodes are provided on a back surface thereof, is applied to an electronic component to cope with the small and narrow connection terminal or the reduced mounting area.
As an electronic component to which the BGA is applied, for example, a semiconductor package is exemplified. In electrodes of the semiconductor package, solder bumps are formed. These solder bumps are formed by joining solder balls in each of which the solder alloy is made spherical to the electrodes of the semiconductor package.
The semiconductor package to which the BGA is applied is mounted on a printed circuit board to which solder paste has been applied with the solder bumps being aligned with the electrodes of the board and by joining the solder paste melted by heating to the solder bumps and the electrodes of the board, the semiconductor package is installed on the board.
As described above, on the solder ball having the principal ingredient of Sn, Sn and O2 in the air may be reacted to form an oxide film by Sn oxides on a surface of the ball. When forming the oxide film on a surface of the solder ball, the phenomenon may occur such that any gloss of the solder ball is lost or a yellow discoloration occurs. As a visual inspection of the solder ball, there is a case where the discoloration of the surface of the solder ball is utilized and unless the discoloration of the surface of the solder ball can be suppressed, it is a strong likelihood to determine that the solder ball is unavailable to use.
In addition, although a flux is generally used when joining in order to remove any oxide film formed during the storage thereof, the wettability deteriorates when the oxide film is insufficiently removed to remain on the surface of the solder ball, thereby causing the fusion properties thereof to deteriorate. A deterioration result of the fusion properties causes, as the phenomenon, any soldering defect to occur.
Although it is conceivable to suppress a growth of the oxide film on the surface of the solder ball by strictly managing the storage conditions thereof, there are a variety of the storage conditions and any issues of the growth of the oxide film and the deterioration of the fusion properties accompanying this are always bothered. Therefore, it is necessary to suppose various kinds of storage conditions to be able to suppress the growth of the oxide film on the surface of the solder ball. Although the solder ball has been described as an example, the issues are common issues of the whole solder having the principle ingredient of Sn in spite of the solder ball.
In order to address the issues, Ge has been generally added to the solder alloy having the principle ingredient of Sn (see, for example, Patent Document 1). In this Patent Document 1, a technology of adding a total amount of 0.006% by mass through 0.1% by mass of one or two or more elements selected from Ge, Ni, P, Mn, Au, Pd, Pt, S, Bi, Sb and In to the solder alloy containing Sn, Ag and Cu, in order to improve the joining property by the soldering, has been disclosed. Patent Document 1 has also disclosed that Ge improves oxidation resistance and Ni, P, Mn, Au, Pd, Pt, S, Bi, Sb and In have effects of lowering the melting point and enhancing joining strength.
In addition, appearance characteristics are also important as quality to be desired in joining by the soldering in addition to secure joining. In the solder alloy having the principal ingredient of Sn, Sn and O2 in the air may be reacted to form Sn oxides which become an oxide film to cover on a surface of the solder alloy. Sn oxides discolor in yellow and the surface of the solder alloy also discolors in yellow, so that there may be a case where it is determined as any failure by the visual inspection.
Accordingly, in order to change optical properties on a surface of the oxide film, a technology of adding a total amount of 1 ppm by mass or more and 0.1% by mass or less of one or two or more elements selected from Li, Na, K, Ca, Be, Mg, Sc, Y, lanthanoids, Ti, Zr, Hf, Nb, Ta, Mo, Zn, Al, Ga, In, Si, and Mn to the solder alloy having the principal ingredient of Sn has been proposed (see, for example, Patent Document 2)